Field of the Invention
The present invention relates to an image reading apparatus, and particularly, to an image reading apparatus including a transmission light source unit for transmission reading of a paper sheet.
Description of the Related Art
Conventionally, an image reading apparatus that reads a paper sheet, such as a bill, includes an image sensor unit that detects light from the paper sheet and converts the light to an electric signal. The image sensor unit applied in the image reading apparatus includes, for example: a linear light source that emits linear light to a paper sheet; and an image sensor that detects reflected light from the paper sheet and converts the reflected light to an electric signal. The image sensor unit emits light to the paper sheet through the linear light source and detects reflected light through the image sensor to perform reflection reading of the paper sheet.
Some image reading apparatus capable of transmission reading of paper sheets include transmission light source units as light source apparatuses for transmission reading. The transmission light source unit includes, for example: a point light source such as an LED; and a rod-like light guide that shapes light emitted by the point light source into a line (shapes a point light source into a line light source). The transmission light source unit is arranged to face the image sensor unit across a conveyance path of the paper sheet. The image sensor of the image sensor unit detects the light emitted by the transmission light source unit and transmitted through the paper sheet and converts the detected light to an electric signal. In this way, the image sensor unit uses the light emitted by the transmission light source unit to perform the transmission reading of the paper sheet.
In the reflection reading, it is preferable that a contrast between the paper sheet and outside around the paper sheet (hereinafter, called background) is high in order to improve accuracy of distinguishing a range (contour of outer shape) of the paper sheet. Particularly, to read a paper sheet with a high brightness, it is preferable that the brightness of the background is lower than that of the paper sheet, and it is more preferable that the brightness is in a black level. However, when the transmission light source unit is arranged to face the image sensor unit, the light emitted by the linear light source of the image sensor unit may enter the light guide of the transmission light source unit, and the light entering the light guide may be emitted toward the image sensor unit. The light will be called re-illumination light. When the re-illumination light enters the image sensor unit, the brightness of a section without the paper sheet (i.e. background) is high in an image output by the image sensor, and the contrast between the paper sheet and the background of the image is reduced.
Patent Document 1
Japanese Laid-open Patent Publication No. 2013-55646
An example of a configuration for preventing the reduction in the contrast between the paper sheet and the background is disclosed in Patent Document 1, wherein a polarizing filter and a light blocking sheet are arranged between the linear light source of the image sensor unit and the light guide of the transmission light source unit. However, the polarizing filter and the light blocking sheet are necessary according to the configuration. Therefore, the number of components is increased, and the configuration is complicated.